GISTEL is a satellitary methodology based on a simple physical model with two components. It is used to estimate global solar irradiance from METEOSAT data. The treated images in the present sequel are of Wefax type of the ＇visible channel＇ of METEOSAT. Our method has been applied in Tunisia. The estimated daily solar global radiation is compared to the measured values performed in three meteorological stations in the North of Tunisia during three successive years. The obtained results have shown that the accuracy level of our method is acceptable for energy applications. Our results show the convenience of GISTEL method to METEOSAT-Wefax images. In general, the relative error does not exceed 20% of the global irradiation. The recorded errors between estimated and measured values are, in some cases, linked to the model＇s parameters. They may also arise from the geographical situation of the site, the satellitary data or from the earth-based solar irradiation.